Automatic arc lighting apparatus and the like



min, 22,, 1949 R. W. @QQCH ZASGI AUTOMATIC ARC LIGHTING APPARATUS AND THE LIKE Filed Oct. 25; 1947 2 Shaw-Sheet l 6e/n icon @611 Pier INVENTOR. Pa/p/r W, 6000;

Nam 22 349 R. W. GOQGH 224839361 AUTOMATIC ARC LIGHTING APPARATUS AND THE LIKE Filed Oct. 25, 194? 2 Sheets-Sheet 2 0,6 IN V EN TOR. WWW! A a /pk W 6006/7 patented Nov. 22, I949 UNITED STATES PATENT OFFICE AUTOPJATIC ARC LIGHTING APPARATUS AND THE LIKE 6 Claims.

The present invention pertains to are lighting and like systems, and more particularly to apparatus for maintaining the electrodes of an are light properly spaced with respect to each other and accurately positioned relative to the reflector.

In arc lighting apparatus for motion picture projection purposes, it is desired to maintain the arc gap constant and at the optical center of the reflector, to avoid variance in the color and intensity of illumination produced by the arc bridging the gap. An object of the present invention is to provide an automatic control system capable of effecting rapid adjustment or feeding of the electrodes with respect to each other, to prevent substantial variations in the characteristics of the electric arc.

Another object of the invention is to provide an automatic control system for a feed motor capable of producing great changes in speed of the motor in response to relatively small variations in voltage in the system from a predetermined value.

A further object of the invention is to provide an automatic control system for a motor operative to feed electrodes with respect to each other, in which relatively small variations in the voltage drop across the electrodes from a predetermined value are capable of effecting a considerable and substantially instantaneous change in the speed of the motor to hold the voltage drop at such predetermined value within close limits.

In its general aspects, the invention includes the provision of a motor for feeding the electrodes relative to one another for the purpose of maintaining the arc gap at a substantially constant figure. The motor has a normal shunt field and a control field, both of which determine the effective excitation of the motor, depending upon the current passing through the fields. With the desired voltage drop across the electrodes, the motor operates at a predetermined speed. However, only slight variation in the voltage drop alters the fiux supplied by the control field greatly with respect to that supplied by the normal field,

causing a large change in the effective excitation of the motor and its corresponding great change in speed. More particularly, the control field is out of phase with the normal motor field, the speed of the motor being determined primarily by the current flowing through the normal field when the voltage drop across the electrodes is at the predetermined value. An increase in such voltage drop to only a very slight extent promptly efiects a comparatively large excitation of the control field, which opposes the flux supplied by the normal field and greatly reduces the effective fiux in the motor, resulting in its very substantial increase in speed to rapidly reduce the arc gap, until the voltage drop is again at the predetermined figure. In general, the control field has very little, if any, efiect on the motor speed when the voltage drop is at the selected value, but only a minor change from such value immediately brings the control field into play to a very con siderable and pronounced extent, to cause a great change in the motor speed.

Several forms which the invention may assume are exemplified in the following description and illustrated by way of example in the accompanying drawings, in which:

ig. 1 is a diagrammatic representation of one system for controlling the feeding of electrodes with respect to each other.

Fig. 2 is a view similar to Fig. 1 of another embodiment of the invention.

Fig. 3 is a diagrammatic view of still a further modification of the invention.

Referring first to the system depicted in Fig. l, a pair of electrodes it, ii are spaced from one another, in order that current fiowing from a suitable D. C. source may bridge the gap between the electrodes and produce the arc. The electrodes may be suitably mounted and shifted with respect to each other by a right and left hand feed screw l2, having suitable gearing l3 secured thereto rotatable by a feed motor it. As the electrodes burn away, the feed motor shifts them toward each other, tending to maintain the arc gap constant, and preferably at the optical center of a r reflector (not shown), when th system employed in connection with a motion picture projection apparatus.

The feed motor t is of the direct current shunt field type, and may draw its armature current and also its normal motor field current from the same D. C. source as the arc. To this end, the armature i5 and field windings iii are connected in parallel across the D. C. source, the field winding preferably being arranged in series with a variable resistor H, in order to control the current flowing through the motor field, and correspondingly determine the speed of the armature I5 in feeding the electrodes iii, H with respect to each other under the influence of the excitation of the motor field it alone.

The direct current feed motor also includes a control field l8, which is wound in opposition to the motor field It, being 180 out of phase therewith. In the system illustrated in Fig. 1, the control field is not excited when the arc gap is at a predetermined value, with the voltage drop across the gap at a corresponding predetermined value. However, upon variation in voltage drop across the electrodes H), H, due to changes in the length of the gap, the control field It is excited to oppose the excitation supplied by the normal motor field l6, thus reducing the effective field flux and causing the motor to increase in speed and efiect feeding of theelectrodes toward one another at a rapid rate to reduce the length of the arc gap.

The control field i8 is excited in dependence upon the conditions existing within a thyratron discharge tube 59, which is normally so arranged that current does not pass through the control field when the voltage drop across the arc is at a predetermined value, but which increases the current flowing through the control field to a maximum extent upon an increase in the voltage drop across the electrodes E, it to only a very minor extent. Accordingly, as will be apparent from Fig. 1, the control field I3 is connected to the plate 29 of the discharge tube, which can be a tctrode gas filled tube, also to the cathode 21 of the tube through a secondary winding 522 of a transformer 23, one end of which is connected to one end of the control field i8 and the other end of which is connected to the cathode 2|. primary transformer winding 2 is connected to a suitable source of alternating current.

The filament of the discharge tube is heated by being connected directly across another secondary winding 25 of the transformer 23.

The tetrode i9 also includes a shield grid 2'! and a control grid '28, which are biased with respect to each other in such manner as to preclude passage of electrons from the cathode 2! to the plate so when the arc voltage is at a predetermined value, but which allows full passage of electrons and the production of a maximum plate current upon increase of the voltage ch. p across the arc gap. Accordingly, the shield grid 21 has negative bias voltage impressed upon it, the shield grid bein connected to a potentiomete' or variable resistor 29, connected directly across a third secondary coil of the transformer through a selenium rectifier 3!, with the usual capacitor 32 connected across thecoil and rectifier to stabilize the rectifier voltage. The arm 33 of the potentiometer is movable across the resistor 2:; to provide a negative bias on the shield grid. 2? of a certain desired value.

The control grid 23 has a positive bias voltage impressed upon it, being connected to the posi tive electrode ii] in series with suitable resistors 34 iunctioning to reduce the control grid voltage with respect to the voltage drop across the arc to th desired figure. A blceder resistor may also be connected across the control grid 28 and cathode iii to stabilize the control grid and hold he thyratron tube in a condition of conductivity. Resistors 35, 3'! are connected in series with the D. C. line and function to reduce the high current present from reaching the tubei9 and other component parts of the circuit. This will prevent any damage to the tube l9 and the circ case short circuit, or failure of any pal t. are also 1 .-ited by the condenser unctions to prevent any A. C. voltage rable firing. Said resistors 3'! allow only voltage to be impressed upon the control id 28.

The system may also include a capacitor connected across the control. field l8 to stabilize 4 the voltage to the field. In addition, if desired, a neon indicator light 45 may be connected across the control field to provide a sensible indication of the operativeness of the system.

As previously indicated, the shield grid 21 has a negative bias voltage impressed upon it through appropriate movement of the potentiometer arm 33, and the control grid 28 has a positive bias voltage impressed upon it. With the predetermined voltage drop across the arc gap, the control grid 28 has a predetermined voltage impressed upon it. The shield grid voltage is so selected through movement of the potentiometer arm as to provide a negative bias voltage of such a value as to prevent firing of the thyratron tube when the control grid voltage is at its predetermined figure, which figure is directly related to the voltage drop across the arc. The lack of firing of the tube obviously results in a zero plate current, and the lack of current flow-- ing through the control field l8. Accordingly, the speed of the shunt wound motor is dependent upon the current flowing through the normal motor field 56, which may be controlled by appropriate adjustment of the variable resistor Assuming that the arc gap between the electrodes l9, ll increases due to their burning away at a more rapid rate than they can be fed toward one another by the feed moto'r I operating unthe excitation of the normal motor field l5 alone, the voltage drop across the gap will in crease, which increases the positive bias on the control grid 28 and causes the gas filled tube to fire, resulting in a production of a maximum plate current, which will flow to the control field l8 and produce its excitation in opposition to the excitation of the normal motor field l6. As a result, the effective excitation of the shunt wound motor is reduced considerably, and its speed is increased considerably to feed the electrodes ID, it toward one another at a rapid rate until the arc gap distance is again at a predetermined value, as determined by the reduction of the voltage drop to its initial figure. When this latter condition obtains, the discharge tube I9 is fired no longer and excitation of the control field i8 ceases, causing an immediate reduction of themotor speed.

It is preferred that the speed of the motor be so chosen as to feed the electrodes l0, ll toward each other at a slower speed than their burning away rate. Accordingly, the control field l8 comes into operation intermittently to increase the speed of the motor and the rate of feed of the electrodes toward one another. Thus, when he arc gap is at the preselected distance, the

normal motor field I6 is operative alone, the 1110- tor speed being too slow and the arc gap immediately beginning to increase the voltage drop thereacross. The tube I9 immediately fires and the control field i8 is excited to increase the motor speed considerably and very rapidly, in fact, practically instantaneously, feeding the electrodes Ill, ll back to the normal gap distance relationship, whereupon the control field I8 is no longer excited, the motor field l6 again being the sole source of excitation, resulting in the motor rotating at too slow a speed, the arc gap again increasing, which brings the control field I8 again into play, and the foregoing cycle of operation repeated. In other words, the control field i8 is intermittently effective at a very rapid'rate to maintain the arc gap distance essentiallyconr, stant.

It is to be noted that the system is so devised and arranged as to cause the control field I8 to be iully operative or inoperative. The firing of the thyratron cube I9 causes a maximum current to pass through the control field, to immediately increase the speed of the motor. When the voltage drop across the arc gap has been reduced to the desired value, excitation of the control field ceases immediately and the motor speed reduces to its former extent. As an example, assuming that the normal voltage drop across the gap is 40 volts and that the motor speed under the influence of the excitation of the normal field I6 alone is 600 R. P. M., an increase in the voltage drop due to an increase in the arc gap to 40.1 volts, or even less, will cause the maximum plate current to flow to the control field I8, such field being excited in opposition to the normal field I6 and increasing the motor speed very rapidly and substantially instantaneously to 2400 R. P. M., or about four times the normal field motor speed, causing a ver rapid feeding of the electrodes I0, II and readjustment of the arc gap distance to the .point at which the voltage drop is again 40 volts.

The system illustrated in Fig. 2 is essentially the same as that shown in Fig. 1, except that the system of Fig. 2 increases the sensitivit of the circuit as a whole in order that the full sensitivity of the tube I9 may be utilized. With this end in mind, the shield grid 21 is connected to the cathode 2i, and the control grid 28 is connected to the positive electrode III through a resistor GI and also to the potentiometer arm 33 through another resistor 42, the potentiometer arm being adjusted with respect to its cooperable resistance as to provide a resultant negative bias on the control grid. For example, if the normal arc potential drop is 40 volts, a lesser positive bias is imposed on the control grid in view of the resistor 4| connected in series therewith. The :potentiometer arm 33, however, is adjusted to offset the positive voltage and provide a resultant negative voltage on the control grid 28 of, for example, volts. When this condition obtains, the

tube I9 does not fire and there is no plate current developed flowing through the control field I8.

Upon increase in the arc voltage drop, due to an increase in the arc gap, the control grid voltage is made less negative, upsetting the balanced condition in the thyratron and causing it to become fully conductive immediately, producing the maximum plate current, which will fiow to the control field and increase the motor speed to the considerable extent indicated above in connection with the system disclosed in Fig. 1. Similarly, the reduction of the arc gap and voltage drop to the desired values results in an increase in the negative bias voltage on the control grid 28 and the inability of the thyratron to continue firing, reducing the control field current to zero, and resulting in a decrease in the motor speed.

With the system illustrated in Fig. 2, variations in A. C. line voltage have no substantial effect on the condition of the thyratron I 9 because a voltage regulator 43 is connected across the potentiometer 29 which holds the voltage across said potentiometer 29 at a constant value regardless of variations in line voltage. By comparison with the system diagrammed in Fig. l, the shield grid 21 in the latter draws very many times the current that the control grid 28 draws in the present system, the first described system possibly being susceptible to undesired firing 0f the tube upon 6 substantial variations in the A. C. line voltage.

Another advantage in connecting the voltage regulator 43 across the potentiometer 29 is that it ofiers greater assurance against changes in the negative voltage impressed on the control grid 28 due to variations in A. C. line voltage. The operation of such regulators is well known in the art.

A simplified system for maintaining the arc gap at a constant figure is illustrated in Fig. 3, in which the electrodes I0, II are connected to a D. C. supply, with the armature I 5, normal motor field I6, and control field I8 being connected in parallel therewith. The line voltage is decreased to provide the desired voltage drop across the arc gap by placing a suitable resistor 44 in series with the circuit, as shown. Similarly, as in the other forms of the invention, the normal motor field I6 may have a variable resistor I'I connected in series therewith to determine the current passing through such field.

The control field I8 is wound in opposition to the normal motor field I6, so that current fiowing through it will result in a decrease in the effective field fiux and produce an increased speed in the motor. The control field is a high impedance field, while the normal motor field I6 is a low impedance field. As an example, the shunt windings of the normal motor field may have 30 ohms resistance, while the control field windings may have 1,000 ohms resistance, the latter having man more turns in order to make the coil much more efiicient in producing flux.

It is preferred that the normal motor field I6 have such an impedance and be so designed as to operate near its flux saturation point, in order that increased current flowing therethrough will have very little, if any, effect upon the excitation supplied by that field. The control field I8, however, operates Well below its saturation point.

With the arc voltage drop at the desired figure, the main flux is supplied by the normal low impedance motor field I6, while comparatively little flux is supplied by the control field I8 in opposition thereto, the motor operating at a speed depending upon the resultant flux supplied by the two field coils. Upon increase in the arc gap, accompanied by an increase in the voltage drop, the current through both normal and control fields I6, I8 is correspondingly increased. However, since the normal motor field I6 is operating at its saturation point, increase in current flowing therethrough effects substantially no change in the flux produced by it. However, the control field I8 is normally operating well below its saturation point, so that an increase in current therethrough produces a substantial increase in the flux supplied by it, particularly since the control coil I8 has many more turns that the normal field coil I 6. Accordingly, an increase in the voltage drop across the arc increases the excitation of the control field I8 far in excess of the normal excitation produced by the shunt wound motor field I6, reducing the effective flux through the motor to a considerable extent and causing its great increase in speed, which will feed the electrodes II), II back toward each other at a very rapid rate and reduce the voltage drop across the electrodes to the initial desired value.

It is, accordingly, apparent that feed motor control systems have been provided which are capable of increasing or changing the feed motor speed to a considerable extent, in order to maintain the arc gap substantially constant. When the electrodes form part of the motion picture projection apparatus, assurance is had that the are gap is held to the predetermined distance and at the optical center of the cooperable reflector. The rapidity with which the control function occurs attests to the extreme sensitivity of the apparatus in promptly modifying the speed of the feed motor in accordance with relatively small changes in voltage drop across the arc gap.

While I have shown several preferred forms of my invention, it is to be understood that various changes may be made therein by those skilled in the art without departing from the spirit-of the invention as defined in the appended claims.

Having thus described my invention, what I claim and desire to secure by Letters Patent 1s:

1. The combination with an automatic motor control for arc welding or the like and including a source of Welding current, fusible electrodes and a motor for feeding the electrodes to maintain a welding arc, of means for controlling the speed of the motor to maintain the arc length sub stantially constant comprising a thyratron tube including a grid, means for applying voltage from the electrodes to the grid, and means for applying a reference voltage of opposite sign on the grid, whereby any change in the arc voltage will appear as an equal change in the grid voltage to vary the excitation of the motor, since the effective voltage appearing at the grid will be equal to the algebraic sum of the two voltages.

2. The combination with an automatic motor control for arc welding current, fusible electrodes and a motor for feeding the electrodes to main-- tain a welding arc, said motor having a field winding and a control field wound in opposition to the first field, of means for controlling the speed of the motor to maintain the arc length substantially constant comp-rising a thyratron tube including a grid and plate, means for connecting the plate to the control field, means for applying voltage from the electrodes to the grid, means for applying a reference voltage of opposite sign to the grid so the effective and critical voltage appearing at the grid will be equal to the algebraic sum of the two voltages, whereby an if} increase in the length or" the are between the electrodes will alter the voltage and grid bias so as to fire the tube and cause a current to flow through the plate and control field to increase the motor speed and feed the electrodes rapidly toward each other.

3. The combination with an automatic motor control for arc welding and the like and including a source of welding current, fusible electrodes, and a motor for feeding the electrodes to maintain a welding arc, said motor having a field winding and a control field wound in opposition to the first field, of means for controlling the speed of the motor to maintain the arc length substantially constant comprising a thyratron tube ineluding a grid and plate, means for connecting the plate to the control field, means including a resistor for applying voltage from the electrodes to the grid, means including a second resistor for applying a reference voltage of opposite sign to the grid so the eifective and critical voltage appearing at the grid will be equal to the algebraic sum of the two voltages and will prevent the tube from firing, an increase in the arc and Voltage between the electrodes altering the voltage on the grid and causing the tube to fire, whereby a current will flow through the plate and control field for speeding up the motor and feeding the electrodes more rapidly.

4. The combination with an automatic motor control for arc welding and the like and including a source of welding current, fusible electrodes, and a motor for feeding the electrodes to maintain a welding arc, said motor having a field winding and a control field wound in opposition to the first field, of means for controlling the speed of the motor to maintain the arc length substantially constant comprising a thyratron tube including a grid and plate, means for connecting the plate to the control field, means including a resistor for applying voltage from the electrodes to the grid, means including a second resistor for applying a reference voltage of opposite sign to the grid so the effective and critical voltage appearing at the grid will be equal to the algebraic sum of the two voltages and will prevent the tube from firing, an increase in the arc and voltage between the electrodes altering the voltage on the grid and causing the tube to fire, wherebya current will flow through the plate and control-field for speeding up the motor and feeding the electrodes more rapidly, the voltage between the electrodes dropping when the electrodes again are spaced the proper distance apart, whereby the grid bias will be such as to stop further firing of the tube and cut oil the flow of plate current to the control field, whereby the motor will return to normal elect-rode feeding speed.

5. The combination with an automatic motor control for arc welding and the like and including a source of welding current, fusible electrodes, and a motor for feeding the electrodes to maintain a welding arc, said motor having a field winding and a control field wound in opposition to the first field, of means for controlling the speed of the motor to maintain the arc length substantially constant comprising a thyratron tube including a control grid, a shield grid and a plate, means for connecting the plate to the control field, means for applying a positive bias on the control grid from the electrodes, means for applying a reference voltage of opposite sign on the control grid 50 the effective and critical voltage appearing on said grid will be equal to the algebraic sum of the two voltages on the control grid, whereby an increase in the length of the are between the electrodes will alter the control grid bias so as to fire the tube and cause a plate current to flow to the control field to increase the motor speed and feed the electrodes rapidly toward each other.

6. The combination with an automatic motor control for arc welding and the like and including a source of welding current, fusible electrodes, and a motor for feeding the electrodes to maintain a welding are, said motor having a field winding and a control field wound in opposition to the first field, of means for controlling the speed of the motor to maintain the arc length substantially constant comprising a thyratron tube including a control grid, a shield grid and a p ate, means for connecting the plate to the control field, means for applying a positive bias on the control grid from the electrodes, means for applying a reference voltage of opposite sign on the control grid so the effective and critical voltage appearing on said grid will be equal to the algebraic sum of the two voltages on the control grid, whereby an increase in the length of the are between the electrodes will alter the control grid bias so as to fire the tube and cause a plate current to flow to the control field to increase the motor speed and feed the electrodes rapidly toward each'other, the voltage between the electrodes dropping when the electrodes again are spaced the proper distance apart, whereby the grid. bias will be such as to stop further firing of the tube and out 011 the flow of plate current to the control field, whereby the motor will return to normal electrode feeding speed.

RALPH W. GOOCH.

REFERENCES CITED The following references are of record in the file of this patent:

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